It was a term made for intersex ppl to describe what happened to them in the past. “I was assigned male at birth.” it was created to show how doctors were factually wrong in the sexing and that they are something entirely different. some trans people started using the term for the same reasons but it quickly devolved into what we have now. It used to be that you WERE agab but now you ARE abag and thats a scientific fact you cant change. thats an entirely different topic on whether hrt functionally changes your sex (it does)